Needle having multiple electrodes

ABSTRACT

Embodiments of the invention generally relate to needles having more than one electrode and which are particularly suited for use in electromyography. One embodiment relates to a tripolar needle having three concentric electrodes. The tripolar needle is formed by providing a needle blank having first and second concentric electrodes and forming a third concentric electrode around the needle blank. The tripolar needle has an outer diameter which is substantially constant along the shaft of the needle, except at the tip, where the needle is sharpened.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/130,222, filed on May 17, 2005, which claims the benefit ofU.S. Provisional Patent Application Ser. No. 60/571,944 filed on May 18,2004, the entire contents of which are hereby incorporated by reference,U.S. Provisional Patent Application Ser. No. 60/571,890, filed on May18, 2004, the entire contents of which are hereby incorporated byreference and U.S. Provisional Patent Application Ser. No. 60/571,942filed on May 18, 2004, the entire contents of which are herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to needles having multiple electrodes. Inparticular, the invention relates to needles having bipolar and tripolarelectrode arrangements which may be used in electromyography.

BACKGROUND

Needles are frequently used in electromyography (EMG) for detectingelectrical signals within the body. Commonly, needles used inelectromyography have one or two electrodes, termed monopolar andbipolar, respectively.

If the diameter of a needle is too large, it can cause significant painto the person in whom it is inserted. Thus it is desirable to have assmall a diameter as possible.

For needles having bipolar electrodes, one of the electrodes is usuallya reference conductor while the other is used as an active conductor. Acommon or ground conductor is placed on the skin externally of the pointat which the body's electrical activity is being monitored. Thisplacement of the common conductor on the skin takes time during the EMGset up procedure. Further, the signal quality achieved with a commonelectrode remote from the measurement point of the active and referenceelectrodes is sub-optimal.

Some needles use a beveled tip. That is, the needle tip is cut at anangle from one side to the other, presenting a substantially ellipticalface in the plane of the cut (assuming that the needle is of circularcross-section). Because of the elliptical shapes of the electrodesexposed on the elliptical face, the distance between the centralelectrode and the outer concentric electrode is not uniform across theexposed elliptical face of the needle at its tip. This lack ofuniformity of distance between the two electrodes can lead toinaccuracies in the signal detection.

A further problem with existing beveled tip concentric needles is thatwhen they are constructed with a small outer diameter, the gap betweenthe active and reference electrodes is often quite small, leading tolower amplitude signals and smaller effective measurement area. There isalso a tendency for the electrodes to short circuit after the tipbecomes worn by repeated insertions. Also, the area of the exposedsurface of the inner conductor is small, resulting in low signalamplitudes.

SUMMARY

The invention relates to a method of forming a needle, comprisingproviding a needle blank having first and second concentric electrodesand an outer insulation layer and forming a third concentric electrodearound the needle blank.

In a still further aspect, the invention relates to a method of forminga needle, comprising providing a needle blank having first and secondconcentric electrodes, forming an outer insulation layer around theneedle blank and forming a third concentric electrode around the outerinsulation layer.

According to these method aspects, it is preferred that the thirdelectrode is formed along a shaft portion of the needle, away from a tipportion of the needle. Further, it is preferred that the needle isformed so that an outer diameter of the third electrode is substantiallythe same as an outermost diameter of the needle blank, which issubstantially the same as an outermost diameter of the needle.

In yet another aspect, the invention relates to a method of forming aneedle, comprising the steps of: providing a needle blank having firstand second concentric electrodes, the first electrode being formedinwardly of the second electrode; removing a portion of the secondelectrode along a shaft of the needle; adding an insulation layer aroundthe needle at least along the part of the shaft from which the portionof the second electrode was removed; providing a conductor layer aroundthe insulation layer; and cutting excess added insulation material fromthe needle so that the outer diameter of the needle is the same as theouter diameter of the needle blank whereby a third concentric electrodeis formed from the conductor layer at least partly along the portion ofthe shaft from which the portion of the second electrode was removed.

In yet another aspect, the invention relates to a method of forming aneedle, comprising providing a needle blank having first and secondelectrodes, providing an insulation material around at least part of theneedle blank and providing a conductive material around at least part ofthe insulation material to form a third electrode.

In one embodiment, the conductive material comprises a conductivesheath, which is preferably in the form of a metallic tube. In anotherembodiment, the conductive material is plated or coated on the outsideof the insulation material.

Preferably, the method includes removing a portion of the needle blankbefore providing the insulation material. The portion removed from theneedle blank is preferably substantially tubular or sheath like.Preferably, the first, second and third electrodes are concentricallyarranged. Preferably, the first electrode is an active conductor and isdisposed inwardly of the second electrode, the second electrode is areference conductor and is disposed inwardly of the third conductor andthe third electrode is a common or ground conductor.

In a preferred embodiment of the method, one end of the needle blank issharpened so as to provide a substantially conical or frustoconicalshape thereto. Alternatively, the needle blank may have one endsharpened so as to provide a beveled tip shape thereto.

Preferably, the needle is formed so as to have an outer diameter along ashaft thereof substantially the same as an outermost diameter of theneedle blank.

In a still further aspect, the invention relates to a method of forminga needle, comprising providing a needle blank having first and secondconcentric electrodes, providing a conductive material around at leastpart of the needle blank and injecting an insulating material betweenthe needle blank and the conductive material.

In another aspect, the invention relates to a needle forelectromyography having first and second electrodes and a sharpened tip,wherein the first and second electrodes are exposed on said tip suchthat the first and second electrodes are separated by a constant gap.

The tip is preferably sharpened so as to have a substantially conical orfrustoconical shape. The first and second electrodes are preferablyconcentrically arranged. The first electrode extends along a centrallongitudinal axis of the needle. The first and second electrodes areseparated by a concentrically arranged insulation layer and the secondelectrode defines an outer diameter of the needle.

In a further aspect, the invention relates to a method of forming aneedle, comprising providing a needle blank having first and secondelectrodes and sharpening one end of the needle blank so as to form asharpened tip and so that the first and second electrodes are exposed onsaid tip such that exposed potions thereof are separated by a constantgap.

Preferably, the method includes sharpening the tip so as to have asubstantially conical or frustoconical shape. Preferably, the first andsecond electrodes are concentrically arranged and are separated by aconcentric insulation layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described in further detail, by wayof example only, with reference to the accompanying drawings, in which:

FIG. 1 is a side cross-sectional view of a tripolar concentric needleaccording to an embodiment of the invention.

FIGS. 2A to 2G are side cross-sectional illustrations of steps in amethod of forming a tripolar concentric needle according to anembodiment of the invention.

FIGS. 3A to 3C are side cross-sectional illustrations of alternativemethod steps for forming a tripolar concentric needle.

FIG. 4 is a side cross-sectional view of a tripolar concentric needleaccording to an embodiment of the invention.

FIGS. 5A and 5B illustrate a tripolar needle in plan and sidecross-sectional views, according to another embodiment of the invention.

FIGS. 6A and 6B illustrate a tripolar needle in plan sidecross-sectional views, according to yet another embodiment of theinvention.

FIGS. 7A and 7B show a bipolar concentric needle having a beveled tip inside cross-section and plan views.

FIG. 8 shows a bipolar concentric needle in side cross-section,according to a further embodiment of the invention.

The drawings are not to scale.

DETAILED DESCRIPTION

Embodiments of the invention generally relate to needles having morethan one electrode and which are particularly suited to use inelectromyography (EMG). Such embodiments of the invention relate tomethods of forming such needles.

Throughout the following description, like reference numerals are usedto indicate like components, features or functions. It should beunderstood that features, components or functions described in relationto one embodiment may also be employed in relation to any otherembodiment, except where it would be apparent to those skilled in theart that such a combination would be unworkable.

The term “proximate” and “proximal” will be used herein to indicate adirection away from the needle tip. The term “distal” will be usedherein to indicate a direction toward, or in the vicinity of, the needletip.

Referring now to FIG. 1, there is shown a side (longitudinal)cross-sectional cylindrical tripolar needle 100. Needle 100 is calledtripolar because it has three conductors, namely a central conductor105, a cannula 115 and an outer conductor 125. Central conductor 105,cannula 115 and outer conductor 125 are electrically connected to an endcap (not shown) engaging a proximal end of the needle 100. In oneembodiment, the electrical connections are made by respective electricalcontacts 107, 117 and 127 by means of an interference fit withrespective contacts in the end cap. Although FIG. 1 indicates thatelectrical contacts 107, 117 and 127 are made by wire connection, thisis less preferred than connection by interference fit.

Central conductor 105 is connected by a wire connection 107 so as to bethe active electrode, while cannula 115 is connected via wire connection117 so as to be the reference electrode. Outer conductor 125 isconnected via wire connection 127 so as to be a common or groundelectrode.

Central conductor 105 is separated from cannula 115 by an innerinsulator 110 surrounding the central conductor 105 in a cylindricalmanner. Cannula 115 is substantially cylindrical and is concentric withcentral conductor 105 and inner insulator 110. Outer insulator 120 isoverlayed on cannula 115, at least along shaft 140 of the needle awayfrom tip 130. Outer conductor 125 substantially cylindrical and isoverlayed on outer insulator 120 and concentric therewith, while alsobeing concentric with cannula 115 and central conductor 105.

Needle 100 has an outer diameter D which is constant along the shaft140. At tip 130, which is substantially conically or frustoconicallyshaped, the diameter of needle 100 focuses toward a point, although dueto physical limitations, the extremity of tip 130 can never truly beformed as a point. At tip 130, central conductor 105, inner insulator110 and cannula 115 have exposed frustoconical portions for contact witha desired part of anatomy during EMG. At tip 130 and along the outerdiameter of needle 100 proximally adjacent tip 130, cannula 115 is theoutermost exposed electrode. More proximally of tip 130, cannula 115reduces in diameter (via a short frustoconical ramp 119) and outerconductor 125 becomes the outermost exposed electrode. Outer insulator120 completely separates cannula 115 from outer conductor 125 at allpoints along shaft 140 and tip 130.

Although not shown in the drawings, needle 100 (and other needleembodiments described herein) has a substantially circular lateralcross-section.

The outer diameter of needle 100 ranges from 0.3 mm to 0.65 mm. Largerdiameter needles are suited for deep anatomical structures or muscleswith large motor units. Small diameter needles are more suited formuscles near the surface with small motor units, such as muscles in thehands and face.

The central conductor 105 is formed of a wire core having a diameter inthe range of 0.025 mm to 0.1 mm. The wire core is formed of platinum,platinum-iridium or other biocompatible metals or alloys.

Cannula 115 has an outermost diameter of 0.3 mm to 0.65 mm and athickness in the range of 0.1 mm to 0.45 mm. The cannula 115 is formedof stainless steel (series 303, 304, 316 or 400), platinum,platinum-iridium, silver or other biocompatible conductive metals oralloys. Cannula 115 may also be plated in gold or gold alloy. Anintermediate diameter of cannula 115 (i.e. where the cylindrical portionhas been removed) is in the range of 0.2 mm to 0.55 mm.

Outer conductor 125 has an outer diameter in the range of 0.3 mm to 0.65mm, which is the same as cannula 115. The radial thickness of outerconductor 125 is in the range of 0.25 μm to 0.4 mm. Outer conductor 125may be formed from stainless steel (series 303, 304, 316 or 400),platinum, platinum-iridium, silver or other biocompatible conductivemetals or alloys. Outer conductor 125 may also be plated in gold or goldalloy.

Inner insulator 110, also called a core insulator, is preferably madefrom epoxy, Teflon or other non-conductive biocompatible materials. Theradial thickness of inner insulator 110 is at least 0.075 mm foreffective insulation.

Outer insulator 120 is preferably formed of epoxy, Teflon or othernon-conductive biocompatible materials. The radial thickness of theouter insulator 120 is at least 0.075 mm for effective insulation.

Tip 130 of needle 100 may be formed as a pencil tip (i.e. generallyconical or frustroconical), such as is shown in FIG. 1, or alternativelyas a beveled tip, such as is shown in FIG. 4. The preferred angle of thetip is about 15 degrees, but it may range from about 5 to 25 degrees.The distance from the tip point to the most distal part of the outerconductor 125 is preferably in the range of 0.2 mm to 10 mm.

The outer diameter of cannula 115 adjacent tip 130 is the same as thatwhich would be used for a bipolar electrode needle. Thus, according to apreferred embodiment of the invention, a tripolar electrode needle canbe obtained without increasing the diameter of the needle.

Advantageously, tripolar embodiments of the invention allow a commonelectrode to be located in close proximity to the active and referenceelectrodes. Further advantageously, the common electrode does not needto be placed on the skin as a separate, time-consuming action because itcan be inserted along with the active and reference electrodes as partof the needle.

Thus, tripolar embodiments of the invention can achieve better signalquality by having the common electrode close to the active and referenceelectrodes, while enabling a more time-efficient EMG set up procedure.This is achieved without increasing the diameter of the needle beyondthat of a standard bipolar needle and therefore without increasing thepain inflicted on the patient.

Referring now to FIGS. 2A to 2G, there are shown side cross-sections ofa needle blank at a number of steps in the formation of needle 100.These steps are part of a method of forming a tripolar needle accordingto one preferred embodiment of the invention.

Referring first to FIG. 2A, there is shown a needle blank 200, havingthe cannula 115 concentrically arranged around inner insulator 110 andcentral conductor 105. Concentric needle blanks such as needle blank 200are commercially available, for example from Excel-tech Ltd. OfOakville, Ontario, Canada.

In FIG. 2B, a generally cylindrical portion of the cannula 115 isremoved along shaft 140. The needle blank with the cylindrical portionremoved is designated by reference numeral 205. A minimum radialthickness of about 75.25 μm is removed from the cannula 115, which isthe minimum thickness of the outer insulator 120 and outer conductor 125combined. The generally cylindrical cannula portion is removed so as toleave a generally frustoconical ramp 119 between the portion of thecannula having been reduced in diameter and the portion of the cannulamaintaining its original diameter. The generally cylindrical portion iscut away removed by, for example, etching, grinding or another machiningprocess of suitable precision.

In FIG. 2C, the needle blank is shown with an added insulation layer toform outer insulator layer 120. This form of the needle blank isdesignated by reference numeral 210. Outer insulator 120 is coated ontoneedle blank 205 so as to cover substantially the whole length thereof.Along shaft 140, the outer insulator 120 is preferably formed around theoutside of cannula 115 so as to have a substantially constant radialthickness less than the outermost diameter of the cannula 115. Thisallows outer conductor 125 to be formed around outer insulator 120 so asto have an outer diameter not greater than that of the outermostdiameter of cannula 115.

Outer insulator 120 is formed by a suitable coating procedure and isallowed to set or cure after it is formed. As an optional step, in orderto ensure that the outer insulator 120 is of constant diameter alongshaft 140, needle blank 210 may be machined so as to cut away any excessinsulator material along shaft 140.

Outer insulator 120 is formed so as to cover ramp 119. Outer insulator120 is therefore formed more thickly adjacent ramp 119 and immediatelyproximally thereof. Outer insulator 120 adjacent ramp 119 serves toseparate cannula 115 at its outermost diameter from outer conductor 125along the outside of needle 100.

In FIG. 2D, outer conductor 125 is applied to needle blank 210,resulting in a form of the needle blank designated by reference 215.Outer conductor 125 is formed by coating, plating, depositing or othersuitable method of forming a layer of conducted material aroundinsulation material.

Outer conductor 125 may be formed around outer insulator 120 withgreater thickness than necessary, as any excess material can be cutaway. The radial thickness of the outer conductor 125 formed along shaft140 must be sufficient to bring the outer diameter of outer conductor125 out to the same diameter as the outermost diameter of cannula 115.

In FIG. 2E, needle blank 215 is cut, machined, etched or otherwisereduced in diameter so as to have a substantially uniform outer diameteralong the length of the needle blank, designated by reference numeral220. In this way, excess conductive and insulative material formed at210 and 215 are removed so as to leave an outer conductor 125 ofsubstantially constant radial thickness along the length of shaft 140and separated from cannula 115 by outer insulator 120. Optionally, ifsufficient thickness of cannula 115 and outer conductor 125 permit, theouter diameter along the shaft and toward the tip (including theoutermost radial part of cannula 115) may be reduced during this cuttingstep by a thickness of, for example, 0.01 mm to 0.1 mm.

In FIG. 2F, tip 130 is formed so as to enable better penetration of theskin, producing a needle blank designated by reference numeral 225. Thetip 130 may be formed by a suitable sharpening method, such as cuttingor grinding, so as to provide a tip having an acute angle of between 5and 25 degrees, and more preferably 15 degrees. Tip 130 is preferablyformed so as to be substantially conical or frustoconical, thusresembling a pencil tip, but may be alternatively formed so as to have abeveled edge (see FIG. 4, for example).

According to one embodiment, only central conductor 105, inner insulator110 and cannula 115 are exposed at tip 130. In an alternativeembodiment, tip 130 may be cut so as to also expose outer insulator 120and outer conductor 125 as part of the tip, rather than only along shaft140.

In FIG. 2G, the needle blank formed at 225 is cut or ground at itsproximal end so as to expose central conductor 105 and cannula 115.Electrical connections 107, 117 and 127 are formed between centralconductor 105, cannula 115 and outer conductor 125, respectively, andthe end cap. The end cap preferably forms the electrical connections107, 117 and 127 by forming a mechanical interference fit withspring-loaded contacts within the end cap. The end cap can then be usedto convey electrical signals picked up by the electrodes in the needleto the EMG equipment. The needle shown in FIG. 2G is designated byreference numeral 230, which corresponds to the final step in formingneedle 100.

In an alternative method of forming needle 100, needle blank designatedby reference numerals 210, 215 and 220 may be formed according toalternative steps, as described hereinafter in relation to FIGS. 3A to3C.

With reference now to FIG. 3A, instead of forming outer conductor 125 bycoating, plating or depositing, a conductive sleeve 325 may be used toform outer conductor 125. After the portion of cannula 115 is removed,conductive sleeve 325 is placed around the outside of the reduceddiameter portion of cannula 115, along the length of shaft 140.Conductive sleeve 325 is positioned so that its distal end is positionedproximally of ramp 119. This form of needle blank is designated byreference numeral 310.

As shown in FIG. 3B, a liquid insulator is injected in betweenconductive sleeve 325 and cannula 115 of needle blank 310. The liquidinsulator is preferably epoxy or Teflon, but may be another form ofsuitable curable material. The outer insulator 120 thus formed byinjection is then allowed to set or cure, which fixes the conductivesleeve 325 in place relative to needle blank 310. This injection andcuring step results in a needle blank designated by reference numeral315.

As shown in relation to FIG. 3C, following the injection of outerinsulator material 120 after curing thereof, excess insulator materialand part of conductive sleeve 325 are removed, for example by machiningit, cutting or etching, and the needle blank is then polished to adesired finish to form needle blank 320. The resulting needle blank 320is of substantially uniform outer diameter and is analogous to needleblank 220.

As an alternative, rather than injecting the insulator fluid betweenconductive sleeve 325 and cannula 115, insulation material may be formedaround the needle blank according to previously described step 210 andconductive sleeve 325 is then slid into position over the insulationlayer while the liquid insulator is sufficiently soft and fluid.

In a further alternative, an outer conductive sleeve may be heated toexpand and then placed over a solid insulator and allowed to cool andcontract. A final cutting step can be used to reduce the outer diameterand to remove excess material.

As an additional step applicable to the above described methods offorming a needle, prior to final formation of needle 100, it ispreferred to polish the needle so as to produce a smooth outer surface.

Referring now to FIG. 4, there is shown a needle 400 of similarconstruction to needle 100, except that it is formed to have a beveledtip 430. Beveled tip 430 is formed by grinding or cutting, for example.Features and functions of needle 400 are otherwise the same as thosedescribed in relation to FIGS. 1, 2A to 2G and 3A to 3C.

Referring now to FIGS. 5A and 5B, there are shown plan and sidecross-sectional views of a further tripolar needle embodiment,designated by reference numeral 500. Needle 500 is substantially similarto needle 100 in form and function, except that it is made with adifferent internal structure.

Needle 500 comprises a central conductor 505, which acts as an activeelectrode, an intermediate conductor 515, which acts as a referenceelectrode, and an outer conductor 525, which acts as a common or groundelectrode. Insulator material 510 separates central conductor 505 fromthe intermediate and outer conductors 515, 525. Outer conductor 525 hasan outer diameter which is the same as the outermost diameter ofintermediate conductor 515. Outer conductor 525 extends generallyproximally of tip portion 530 and intermediate conductor 515.

Central conductor 505, insulator 510 and intermediate conductor 515 areexposed at a pencil tip shaped tip portion 530. Central conductor 505comprises an enlarged head portion towards tip 530, which is wire bondedto a wire conductor extending within shaft 540 back to a proximal endcap (not shown). Intermediate conductor 515 also has an enlarged headportion at tip 530, extending roughly cylindrically and frustoconicallytoward the pencil tip shaped end portion 530. Intermediate conductor 515is also wire bonded to a wire conductor extending proximally along shaft540 to the end cap.

Referring now to FIGS. 6A and 6B, a further tripolar needle embodimentis shown in plan and side cross-section, respectively. Needle 600 issubstantially similar to needle 500, except that it has a beveled tip630 instead of the pencil shaped tip 530. Reference indicators used inrelation to FIGS. 6A and 6B correspond generally to those parts shownand described in relation to FIGS. 5A and 5B, where the last two numbersof the reference numerals are the same. For example, outer conductor 625corresponds to and has substantially the same form and function of,outer conductor 525.

Referring now to FIGS. 7A and 7B, there is shown a concentric bipolarneedle 700 having a beveled tip 730. Needle 700 has a central conductor705 and an outer conductor 715 separated by insulator 710. Needle 700presents an elliptical face at its tip, where the exposed end surface ofcentral conductor 705 is located in the center of the exposed face.Insulator 710 separates central conductor 705 from outer conductor 715by a variable distance X according to the position on the exposedelliptical face. The distance X is largest in the direction oflongitudinal extension of the needle 700, while it is smallest in thelateral direction, as is evident in FIG. 7B. The exposed elliptical facecan be used by a medical practitioner conducting the EMG todirectionally target a signal source.

Referring now to FIG. 8, a further embodiment of the invention is shownin side cross-section as needle 800. Needle 800 has a central conductor805 and an outer conductor 815 separated by an insulator 810. A tip 830of needle 800 is formed so as to have the shape of a pencil tip, ratherthan a beveled tip. Forming the tip 830 of bipolar concentric needle 800as a pencil tip allows for a distance Y between central conductor 805and outer conductor 815 to be substantially constant around the tip. Thepreferred dimensions and materials for needle 800 are the same as forneedle 100, where outer diameter 815 equates to cannula 115.

Needle 800 provides a better signal quality than needle 700, resultingfrom the constant distance between the active and reference electrodes.This distance can be sized to match the typical motor unit being probed,with different tips for different muscles. The constant distance betweenelectrodes also prevents electrical breakdown due to an inadequateinsulation gap that can occur with beveled tip geometries.

Needle 800 also provides enhanced patient comfort as the gauge of theneedle can be made smaller than conventional concentric needle designs.This is because the same insulation gap as that of a standard beveledtip concentric needle can be achieved with a smaller diameter using apencil tip. The insulation gap can be further increased by making theangle of the pencil tip more acute, say in the order of 10 degrees.

While the present invention has been described with reference to whatare presently considered to be the preferred examples, it is to beunderstood that the invention is not limited to the disclosed examples.To the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the invention.

1. A method of forming a needle, comprising: providing a needle blankhaving first and second concentric electrodes; providing an insulationmaterial around at least part of the needle blank; providing aconductive material around at least part of the insulation material toform a third electrode; and sharpening one end of the needle blank toform a sharpened tip portion of the needle.
 2. The method of claim 1,wherein the conductive material is plated or coated on the outside ofthe insulation material.
 3. The method of claim 1, wherein theconductive material comprises a conductive sheath of a substantiallytubular form.
 4. The method of claim 1, further comprising the step ofremoving a portion of the second electrode before providing theinsulation material.
 5. The method of claim 1, wherein the step ofsharpening provides a substantially conical or frustoconical shape atthe one end of the needle.
 6. The method of claim 1, wherein the step ofsharpening provides a beveled tip shape at the one end of the needle. 7.The method of claim 1, wherein the third electrode is formed along ashaft portion of the needle, proximally of the tip portion.
 8. Themethod of claim 1, wherein the needle is formed so that an outerdiameter of the third electrode is substantially the same as anoutermost diameter of the needle blank, which is substantially the sameas an outermost diameter of the needle.
 9. A method of forming a needle,comprising: providing a needle blank having first and second concentricelectrodes; providing a conductive material around at least part of theneedle blank; injecting an insulating material between the needle blankand the conductive material; and sharpening one end of the needle blankto form a sharpened tip portion of the needle.
 10. The method of claim9, wherein the conductive material comprises a conductive sheath of asubstantially tubular form.
 11. A method of forming a needle,comprising: providing a needle blank having first and second concentricelectrodes; forming an outer insulation layer around the needle blank;forming a third concentric electrode around the outer insulation layer;and sharpening one end of the needle blank to form a sharpened tipportion of the needle.
 12. The method of claim 11, further comprisingthe step of removing some of the outer insulation layer and some of thethird electrode so that an outermost diameter of the needle issubstantially constant along a shaft of the needle.
 13. The method ofclaim 12, wherein the outermost diameter of the needle is between 0.3 mmand 0.65 mm.
 14. The method of claim 9, wherein the third electrode isformed along a shaft portion of the needle, proximally of and adjacentto the tip portion.
 15. The method of claim 12, wherein the needle isformed so that an outer diameter of the third electrode is substantiallythe same as an outermost diameter of the needle blank, which issubstantially the same as the outermost diameter of the needle.
 16. Themethod of claim 9, wherein the third electrode is plated or coated onthe outside of the insulation layer.
 17. The method of claim 9, whereinthe third electrode comprises a conductive sheath of a substantiallytubular form.
 18. The method of claim 12, further comprising the step ofremoving a portion of the second electrode before forming the outerinsulation layer.
 19. The method of claim 11, wherein the step ofsharpening provides a beveled tip shape at the one end of the needle.20. The method of claim 11, wherein the step of sharpening provides asubstantially conical or frustroconical tip shape at the one end of theneedle.
 21. The method of claim 20, wherein the tip is sharpened so thatthe first and second electrodes have respective exposed portions on thetip separated by a substantially constant gap.
 22. A method of forming aneedle, comprising the steps of: providing a needle blank having firstand second concentric electrodes, the first electrode being formedinwardly of the second electrode; removing a portion of the secondelectrode along a shaft of the needle; adding an insulation layer aroundthe needle at least along the part of the shaft from which the portionof the second electrode was removed; providing a conductive material toform a conductive layer around the insulation layer; cutting excessadded insulation material and conductive material from the needle sothat an outer diameter of the needle is the same as an outer diameter ofthe needle blank, whereby a third concentric electrode is formed fromthe conductor layer at least partly along the portion of the shaft fromwhich the portion of the second electrode was removed; and sharpeningone end of the needle blank to form a sharpened tip portion of theneedle.